Engine coolant heaters are commonly used during winter in the colder parts of the country. The heater maintains a vehicle engine warmer than without the heater overnight and thus provides easier starting and faster warmup.
A typical heater uses an electrical resistance element mounted mostly to one side of a circular base. The base is attached to the exterior of an engine block over an opening connected to a coolant passage of the engine. All liquid cooled engine blocks have several of these openings which are covered by circular plates or freeze plugs. If the engine coolant freezes and expands, the freeze plugs are designed to be pushed out of the opening and thus relieve the expansive pressure and prevent block cracking.
The typical heater element has two parallel legs connected by a return bend portion. Also, the legs are bent or curved so that the return bend portion and a substantial portion of the legs are inserted through and past the block opening and actually into the coolant passage. This works well as long as the coolant passage is wide enough to allow the heating element to enter. However, it is known that many modern engines do not have wide enough coolant passages.